Self-propelled irrigation systems of the center pivot type have been in widespread use in the industry and are generally characterized by having an elongated boom rotatably attached at one end to a supply pipe located at the center of an area to be irrigated. The boom is supported by a plurality of wheeled support towers, each tower successively away from the center being driven at a greater rate of speed in order to maintain the boom in a substantially straight line as it rotates about the center. End or corner booms have also been proposed which can be articulated with respect to the main support towers in order to cover irregular areas of terrain beyond the periphery of the main support tower sections. Almost without exception such systems require fairly sophisticated controls, such as, by the utilization of buried wires or guide tracks which will cause the end boom to follow a predetermined path of travel around the outer periphery of the area to be irrigated; or programmed circuitry is employed to regulate the movement of the end boom relative to the main tower sections. Representative of such approaches taken in the prior art is U.S. Pat. No. 3,352,493 to Curtis which employs an end gun controlled by a solenoid, the solenoid being operated in accordance with the radial position of the water supply conduit. U.S. Pat. No. 3,608,826 to Reinke also employs an end gun with articulated pipe sections which are driven by wheeled support towers in a circular path and are controlled to cover irregular boundaries along the outer periphery of the area to be irrigated. U.S. Pat. No. 3,802,627 to Seckler employs a center pivot arrangement with an outer wheeled support tower or extension boom which is electrically controlled to advance independently of the main boom or tower sections to irrigate sections outside of the circular periphery traversed by the main support towers. Seckler accomplishes same through the utilization of a position angle encoder, such as, an analog-to-digital instrument in order to represent the position angle of the wheels of the outermost support tower with respect to an arbitrarily selected reference angle and, in combination with a stress switch, will energize a driving motor for the outermost tower.
Important to recognize in such systems with an articulated end boom assembly is that the wheeled supports must traverse a different arc or path of travel which imposes requirements not only in terms of relative speed of travel but steering of the wheels. In Seckler, for example, an independent type of a crankshaft steering mechanism is employed under the control of a steering motor for the steering wheels of the outermost tower for the extension boom. A similar approach to the same problem is disclosed in U.S. Pat. No. 3,902,668 to Daugherty in which the outermost boom support tower is pivotally connected to the end of the main water conduit and the wheels of the outermost tower are driven in a direction determined by an electrical sensor which is field-coupled to a conductor which establishes the path along which the end support tower is to advance. Again, however, the wheels of the outermost boom support tower are steered by a sensor which is attached to the boom and field-coupled to a buried electrical conductor. Other representative patents are U.S. Pat. Nos. 3,539,107 to Mitchell and 3,281,080 to Hogg for irrigation systems utilizing wheeled support towers.
It is desirable in accordance with the present invention to avoid the requirement for special steering mechanisms for the end boom assembly as well as the need for buried electrical wires or guide tracks which are expensive and costly to install and maintain as well as to overcome definite limitations with respect to satisfactory operation under varying conditions of use and types of irrigation systems employed.